<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        const str = "<p class='demo'>hello JavaScript</p>";
        const reg = /^\<.+\>(.+)\<.+\>$/;
        const str1 = str.replace(reg,function (match, $1) {
            return $1;
        });
        console.log(str1)
    </script>
</body>
</html>